The present invention relates to a coextrusion binder composition and to coextruded multilayer composites in which the said composition is employed as adhesive layer.
Coextruded multilayer composites which have an outer layer of polystyrene, polycarbonate or polyester, especially polystyrene, are commonly employed for food containers such as cups and containers of the yogurt or compote pot type and the like, or for films such as thermoformed or heat-sealed lids. However, in these composites the above-mentioned outer layer is generally lined with a layer of ethylene-vinyl alcohol copolymer, polyamide, polyesteramide, polyolefin and the like, or of a mixture of these polymers, which have a poor affinity for it, with the result that provision must be made for an adhesive layer between the two. By way of examples of multilayer composites there may be mentioned those containing three layers (for example polyethylene/binder/polystyrene), or five layers (for example polyethylene/binder/ethylene-vinyl alcohol copolymer/binder/polystyrene) and the like.
The present invention also includes the composites formed by hot lamination or resin-coating, a technology according to which a multilayer including the binder is extruded over a sheet of poly(methyl methacrylate), polystyrene, poly(ethylene terephthalate) and the like, the binder providing especially the adhesion to the sheet.
Many adhesive formulations have been developed for this purpose and are described in the literature. Most of the time these formulations include an ethylene-vinyl acetate copolymer grafted with maleic anhydride and/or styrene, in combination with another component which is especially an ungrafted ethylene-vinyl acetate copolymer, polystyrene, impact polystyrene, a petroleum resin and the like. The compositions described in U.S. Pat. Nos. 4,861,676 and 4,861,677 may be mentioned, among others.
French Patent Application No. 2 677 658 also discloses an adhesive composition obtained by grafting an unsaturated carboxylic acid or a derivative thereof onto a mixture including 40 to 95% by weight of ethylene-(meth)acrylic ester copolymer which has a (meth)acrylic ester content of 25 to 45% by weight, 5 to 30% by weight of polystyrene and 0 to 30% by weight of a polymer which may be an ethylene-vinyl acetate copolymer.
While, among the many resins presented in the literature, some are satisfactory from the viewpoint of adhesive performance, the latter must still be improved. In addition, the multilayer composites very often do not cut out properly. Such cutting out, generally carried out after the thermoforming or after the filling of the containers and fitting the lid, is performed either by pressure of a metal xe2x80x9cnetxe2x80x9d on a table, or by shearing. If the cutting out is not correct, the thermoformed article remains attached to the sheet from which it has been formed. This gives rise to uncontrolled production stoppages and the changing of the cutter blades.
We have therefore investigated compositions which have adhesive performances that are correct, or even superior to those of the known compositions, but which can offer the additional advantage of allowing a greater ease of cutting out than the latter.
These compositions include:
at least one out of
(A1) the graft polymers resulting from the grafting of at least one grafting monomer such as maleic anhydride onto (a) impact or crystal styrene homopolymers and copolymers and/or (b) optionally hydrogenated styrene-diene elastomer block polymers, (b) not being the only polymer in the mixture of (A) and (B);
(A2) at least one copolymer (b) mixed with at least one polymer (B1);
(A3) graft polymers resulting from the cografting of at least one grafting monomer onto a mixture of at least one polymer (a) and of at least one of (c) ethylene-vinyl acetate copolymers, ethylene-xcex1-alkyl (meth)acrylate copolymers, ethylene homopolymers, and ethylene-xcex1-olefin copolymers;
(A4) graft polymers resulting from the cografting of at least one grafting monomer onto at least one polymer (a) to which at least one tackifying resin (d) has been added, these graft polymers being mixed with at least one polymer (B1); and optionally
at least one out of
(B1) graft polymers resulting from the grafting of at least one grafting monomer onto a polymer (c); and
(B2) (ethylene-xcex1-olefin or vinyl acetate or alkyl (meth)acrylate-monomer of the type of the above-mentioned grafting monomers) terpolymers; and
(C) the polymers (a), (b), and (c).
In accordance with the present invention, a coextrusion binder composition is proposed first of all, characterized in that it includes:
at least one polymer (A) chosen from:
(A1) graft polymers resulting from the grafting of at least one grafting monomer chosen from carboxylic acids containing ethylenic unsaturation, the corresponding acid anhydrides and the derivatives of these acids and acid anhydrides, onto
(a) impact or crystal styrene homopolymers and copolymers; and/or
(b) styrene-diene elastomer block polymers and these same copolymers in the hydrogenated state, provided that (b) is not the only polymer in the mixture of (A) and optionally (B);
(A2) at least one copolymer (b) as defined above mixed with at least one polymer (B1) as defined below;
(A3) graft polymers resulting from the cografting of at least one grafting monomer as defined above onto a mixture:
of at least one polymer (a) as defined above; and
of at least one out of (c) ethylene-vinyl acetate copolymers, ethylene-alkyl (meth)acrylate copolymers, ethylene homopolymers and ethylene-xcex1-olefin copolymers, provided that the ethylene-alkyl (meth)acrylate copolymers may not represent more than 40% by weight of the mixture subjected to the cografting in the case of a polystyrene content lower than 30% by weight if (A3) represents the major constituent of the binder composition;
(A4) graft polymers resulting from the cografting of at least one grafting monomer as defined above onto at least one polymer (a) as defined above, to which at least one tackifying resin (resin possessing adhesive bondability) (d) has been added, these graft polymers being furthermore mixed with at least one polymer (B1) as defined below; and optionally
at least one out of:
(B) the polymers chosen from:
(B1) graft polymers resulting from the grafting of at least one grafting monomer as defined above onto a polymer (c) chosen from ethylene-vinyl acetate copolymers, ethylene-xcex1-olefin copolymers, ethylene-alkyl (meth)acrylate copolymers, ethylene homopolymers; and
(B2) (ethylene-xcex1-olefin or vinyl acetate or alkyl (meth)acrylate-monomer of the type of the abovementioned grafting monomers) terpolymers; and
(C) the polymers (a), (b) and (c) as defined above.
In general, each of the graft polymers (A1), (B1), the cografted copolymers (A3) and all the polymers comprising a cografted resin (d), included within the formulations of the compositions of the invention, comprise from 0.005 to 5% by weight of units originating from the grafting monomer(s) relative to the polymer of to the mixture of polymers subjected to grafting. The grafting monomers are chosen especially from (meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, crotonic acid, itaconic anhydride, maleic anhydride or a substituted maleic anhydride, such as dimethylmaleic anhydride or else a salt, amide, imide and ester of a carboxylic acid containing ethylenic unsaturation, such as mono- and disodium maleate, acrylamide, maleimide and diethyl fumarate. Maleic anhydride and maleic acid are preferred.
The grafting may be performed by a known method consisting in melting the polymer(s) to be grafted, adding thereto the grafting monomer and from 50 to 20 000 ppm, relative to the polymer(s) of a radical polymerization initiator, mixing so as to obtain a uniform distribution of the grafting monomer and of the initiator in the polymer(s), kneading the resulting mixture in an extruder at a temperature above the melting point of the polymer(s), extruding the resulting graft polymer as a shaped article, tablets or other forms which are subsequently employed as such or as a mixture with other polymers, with a view to the coextrusion of multilayer structures, as will be described below.
Apart from this grafting in an extruder, another possible method which may also be mentioned is grafting in solution, consisting in dissolving the polymer(s) to be grafted in a solvent and adding thereto the grafting monomer(s) and the initiator to perform the graft polymerization at a temperature of between 80 and 150xc2x0 C.
The free radical initiator may belong to different classes which are well known to a person skilled in the art. Among these there may be mentioned peroxides, peresters, hydroperoxides and diazo compounds. Peroxides which may be mentioned are dicumyl peroxide, di-tert-butyl peroxide, benzoyl peroxide, lauroyl peroxide, xcex1,xcex1xe2x80x2-bis(t-butylperoxy-m-isopropyl)benzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne. Tertiary butyl perbenzoate may be mentioned as perester, t-butyl hydroperoxide and cumene hydroperoxide as hydroperoxide, and azobisisobutyronitrile as diazo compound.
If reference is made to the general definition of the compositions according to the invention it may be seen that a grafting may be performed, for example on a polystyrene, and then, if appropriate, the resulting graft polymer may be xe2x80x9cdilutedxe2x80x9d with at least one other polymer, which itself may be grafted. Another solution consists in performing a cografting, for example, on a polystyrene and an ethylene-vinyl acetate copolymer, which offers the advantage of simplifying the manufacture, since the required composition is obtained directly if the cografting is not followed by a dilution.
The polymers (a) included within the composition according to the invention include styrene homopolymers and copolymers (crystal polymers) and polystyrenes containing rubbery components (impact polymers), in particular those referred to as high-impact.
Examples of styrene copolymers which may be mentioned are chloropolystyrene, poly-xcex1-methylstyrene, styrene-chlorostyrene copolymers, styrene-propylene copolymers, styrene-butadiene copolymers, styrene-isoprene copolymers, styrene-vinyl chloride copolymers, styrene-vinyl acetate copolymers, styrene-alkyl acrylate (methyl, ethyl, butyl, octyl, phenyl acrylate) copolymers, styrene-alkyl methacrylate (methyl, ethyl, butyl, phenyl methacrylate) copolymers, styrene-methyl xcex1-chloroacrylate copolymers and styrene-acrylonitrile-alkyl acrylate copolymers. The comonomer content in these copolymers generally ranges up to 20% by weight.
The polystyrenes containing rubbery components are especially those containing from 1 to 20% by weight of units originating from the rubbery components which are, for example, butadiene and isoprene.
The copolymers (b) are the above-mentioned block copolymers obtained by copolymerization of styrene and of a diene such as butadiene and isoprene. Diblock styrene-butadiene and styrene-isoprene and triblock styrene-butadiene-styrene and styrene-isoprene-styrene copolymers may be mentioned. Their preparation enables linear or branched products to be obtained. These same copolymers may be hydrogenated. SEBSs or SEPSs are thus obtained, depending on whether SBSs or SISs are employed as base for the hydrogenation. The styrene content of these copolymers is lower than or equal to 50% by weight. The styrene/diene weight ratios are especially between 10/90 and 50/50.
The polymers (c), also included within the definition of the component (A3), are especially:
ethylene-vinyl acetate copolymers which have a vinyl acetate content generally lower than 60% by weight, especially lower than 50% by weight;
ethylene homopolymers and copolymers of ethylene and of at least one alpha-olefin (all copolymers commonly denoted by PE, LDPE, LLDPE, VLDPE, HDPE and EPR). The alpha-olefin generally contains 3 to 12 carbon atoms and is chosen especially from propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and their mixtures. The content of units derived from ethylene is generally at least 40% by weight and the relative density of these homo- and copolymers is generally between 0.880 and 0.970;
ethylene-alkyl, in particular C1-C12 alkyl, (meth)acrylate copolymers which have a (meth)acrylate content generally of between 5 and 60% by weight, preferably between 7 and 40% by weight; methyl, ethyl and butyl (meth)acrylates may be mentioned as alkyl (meth)acrylates.
Furthermore, a quantity of at least one resin which has an adhesive bondability (d) may be added to at least one of the polymers or mixtures of polymers subjected to a grafting or to a cografting, in order to lead to a grafting or cografting product included within the composition of the invention.
As tackifying resins (d) which may be included within the composition of the coextrusion binder according to the invention in the cografted state, there may be mentioned styrene-based resins such as poly-xcex1-methylstyrene resins, rosin resins, rosin ester resins, aliphatic petroleum resins and terpene, terpene-phenolic, coumarone and coumarone-indene aromatic resins. These resins generally cannot represent more than 40% by weight of the composition. The content of these tackifying resins in the composition, which depends on the required melt index, does not preferably exceed 10% by weight. Furthermore, some may be hydrogenated, such as rosin resins, aliphatic petroleum resins and terpene resins.
Compositions according to the invention which have been found particularly advantageous are the following (the total composition representing 100% by weight each time):
compositions consisting of:
5 to 70% by weight, in particular 5 to 45% by weight, of polystyrenes consisting of at least one impact or crystal polystyrene grafted with optionally ungrafted impact polystyrene, it being possible for the latter to represent up to 40% by weight of the composition; and
30 to 95% by weight, in particular 55 to 95% by weight, of an ethylene-vinyl acetate graft copolymer or of an ethylene-alkyl (meth)acrylate graft copolymer or of an (ethylene-vinyl acetate or alkyl (meth)acrylate-maleic anhydride) terpolymer, the grafts being grafts with maleic anhydride,
optionally mixed with impact or crystal polystyrene and/or an ethylene-vinyl acetate copolymer and/or an ethylene-alkyl (meth)acrylate copolymer;
compositions consisting of the product of cografting of maleic anhydride onto a mixture of:
5 to 70% by weight, in particular 5 to 45% by weight, of at least one impact or crystal polystyrene; and
30 to 95% by weight, in particular 55% to 95% by weight, of an ethylene-vinyl acetate copolymer and/or of an ethylene-alkyl (meth)acrylate copolymer, the condition indicated in the case of (A3) being furthermore complied with,
optionally mixed with impact or crystal polystyrene and/or an ethylene-vinyl acetate copolymer and/or an ethylene-alkyl (meth)acrylate copolymer;
compositions consisting of:
10 to 40% by weight of the product of cografting of maleic anhydride onto a mixture of impact polystyrene and of a minor quantity of an alpha-methylstyrene tackifying resin; and
60 to 90% by weight of ethylene-vinyl acetate graft or ethylene-methyl (meth)acrylate graft copolymer; and
compositions consisting of:
10 to 30% by weight of a styrene-butadiene-styrene block copolymer; and
70 to 90% by weight of ethylene-vinyl acetate copolymer grafted with maleic anhydride.
The present invention also relates to a coextruded multilayer composite including at least once the succession of the following three layers:
a layer of polystyrene or of polycarbonate or of polyester,
a layer of the coextrusion binder composition as defined above; and
a layer consisting of at least one resin chosen from polyolefins, ethylene-vinyl alcohol copolymers, polyamides and polyesters.
The following combinations may be mentioned in particular:
PS/binder/EVOH, PS/binder/PA, PS/binder/PET, PS/binder/PO, PS/binder/EVOH/binder/PS, PS/binder/EVOH/binder/PO, PS/binder/EVOH/binder/PET and PS/binder/EVOH/PA/binder/PS, where PS=polystyrene, EVOH=ethylene-vinyl alcohol copolymer, PA=polyamide, PET=poly(ethylene terephthalate) and PO=polyolefin.
The polystyrenes PS are as defined above.
Polyolefins PO here include an ethylene-xcex1-olefin copolymer, an ethylene-vinyl acetate copolymer, ethylene-alkyl (meth)acrylate copolymers which have an alkyl (meth)acrylate content lower than 25% by weight, low-density polyethylenes, high-density polyethylenes, polypropylenes and mixtures of these polymers.
Ethylene-vinyl alcohol EVOH copolymers include saponified ethylene-vinyl acetate copolymers which have a degree of saponification of at least 50 mol %. It is preferable that these polymers should contain at least 30 mol % of vinyl alcohol units, to obtain good adhesiveness and gas impermeability properties.
The polyamides PA are linear polymers containing acid amide bonds, obtained by condensation of diamines and dicarboxylic acids, condensation of aminoacids or decyclization of lactams. Representative examples of these polyamides are nylon 6, nylon 6-6, nylon 6-10, nylon 11 and nylon 12.
The polyesters employed in the coextruded composites of the present invention are polymers obtained by condensation of saturated diacids and of glycols. It is possible to mention in particular poly(ethylene terephthalate) obtained from ethylene glycol and terephthalic acid, poly(ethylene terephthalate) copolymers which have, as copolymerization component, a saturated diacid such as phthalic acid, isophthalic acid, sebacic acid, adipic acid, azelaic acid, glutamic acid, succinic acid, oxalic acid and the like, poly(ethylene terephthalate) copolymers which have, as copolymerization component, a diol such as 1,4-cyclohexanedimethanol, diethylene glycol, propylene glycol and the like, and mixtures of these polymers.
The thickness of the composite of the present invention is from 100 xcexcm to 3 mm, preferably 500 xcexcm to 2 mm. In the case of the composite containing three layers of polystyrene/binder/sealing material (such as a low-density polyethylene, a linear low-density polyethylene, an ethylene-vinyl acetate copolymer and the like), the thickness of the composite is from 100 to 1000 xcexcm, preferably 100 to 600 xcexcm. The thickness of the polystyrene layer in the composite according to the present invention represents 30 to 96%, preferably 60 to 90%, of the total thickness of the laminate. The thickness of the binder composition layer represents 1 to 35%, preferably 2 to 20%, of the total thickness of the composite, and the thickness of the sealing material layer represents 1 to 35%, preferably 2 to 20% of the total thickness of the composite.
The peel strength of the composites of the present invention, in the case where the thickness of the binder layer is 30 xcexcm, is higher than 6 N/15 mm, generally at least 9 N/15 mm.
The binder compositions according to the present invention and the multilayer composites employing these compositions have excellent properties of adhesive strength and of resistance to the separation of the layers after coextrusion, as well as excellent cutting-out properties, with the result that they are very useful for food containers and the like.
The following examples illustrate the invention without, however, limiting its scope. In these examples the percentages are given by weight unless indicated otherwise. The particular compositions of coextrusion binders used in these examples belong to types II to IX, detailed in Table 1 below, type I representing the reference composition. In this table the constituents are indicated, the following abbreviations being employed:
Furthermore, the particular constituents below were used in these examples (see also first column of Table 2). The constituent unit contents of the various graft copolymers are here weight contents measured by FTIR spectroscopy; the melt indices, expressed in g/10 min, were measured according to ASTM standard D-1238 in condition (L) (23xc2x0 C., 2.16 kg) in the case of impact PS, crystal PS and in condition (E) (190xc2x0 C., 2.16 kg) in the case of impact PS g MA, crystal PS g MA and other copolymers and terpolymers; the Izod impact strength was measured according to ISO standard 180/1A, and the Vicat softening temperature according to ISO standard 306 B.
The melting points were obtained according to the ATD method.
Individual Constituents Used
The impact PSs 1, 2, 3 and 4 and the crystal PS employed in the examples are polystyrenes marketed under the series name xe2x80x9cLacqrxc3xa8nexe2x80x9d by the company Elf Atochem S.A. Their characteristics are recalled in Table 2 below:
Preparation of Impact PS 1 g MA (Lacqrxc3xa8ne 8350 g MA)
Lacqrxc3xa8ne 8350 impact polystyrene and maleic anhydride, in a ratio of 1.5% by weight relative to the impact polystyrene, are introduced into a corotative twin-screw extruder of Werner 30 type (12 barrels, 30 mm diameter). 8500 ppm of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (Trigonox 101) are injected sideways into the extruder at barrel No. 4. The barrel temperatures displayed are between 180xc2x0 C. (barrel No. 1) and 210xc2x0 C. (barrel No. 10). The stock temperature is approximately 200xc2x0 C. The residues of free maleic anhydride are removed by degassing at barrel No. 10. The overall throughput of the extruder is 20 kg/hour and the speed of rotation of the screws 280 rev/min. Impact polystyrene grafted with 0.9% of maleic anhydride and exhibiting a melt index of 3.2 is obtained.
Preparation of Impact PS 2 g MA (Lacqrxc3xa8ne 7240 g MA)
The procedure is as for the preparation of impact PS 1 g MA, except that impact PS 2 is employed. The polymer obtained is an impact polystyrene grafted with 0.96% of maleic anhydride and exhibiting a melt index of 3.3.
Preparation of Impact PS 3 g MA (Lacqrxc3xa8ne 5240 g MA)
The procedure is as for the preparation of impact PS 1 g MA, except that impact PS 3 and 15 000 ppm of Trigonox 101 are employed. The polymer obtained is an impact polystyrene grafted with 1.2% of maleic anhydride and with a melt index of 3.1.
Preparation of Crystal PS g MA (Lacqrxc3xa8ne 1160 g MA)
The procedure is essentially as for the preparation of impact PS 1 g MA, except that Lacqrxc3xa8ne 1160 crystal PS is employed. The proportion of maleic anhydride introduced is 1.5% by weight, and 1.1% by weight of Trigonox 101 is injected sideways, these quantities being in relation to the crystal PS. The speed of rotation of the screws is 130 rev/min and the throughput is 10 kg/hour. Crystal polystyrene grafted with 0.98% of maleic anhydride, with a melt index of 3.3, is obtained.
Preparation of EVA copo g MA
Into the hopper of a corotative twin-screw extruder of Werner 30 type (see above) are introduced: an ethylene-vinyl acetate copolymer (vinyl acetate content: 28% by weight, melt index: 4), maleic anhydride in a proportion of 0.25% relative to the EVA copolymer, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (Luperox 101) in a proportion of 200 ppm relative to the EVA copolymer. The displayed temperatures of the barrels are between 170xc2x0 C. (barrel No. 1) and 220xc2x0 C. (barrel No. 8). The stock temperature is approximately 200xc2x0 C. The residues of free maleic anhydride are removed by degassing at barrel No. 10. The overall throughput of the extruder is 15 kg/hour. An EVA copolymer grafted with 2000 ppm of maleic anhydride is obtained, exhibiting a melt index of 3 to 3.5, a melting point of 75xc2x0 C. and a Vicat softening point of 51xc2x0 C.
Preparation of EMA copo g MA
Into the hopper of a corotative twin-screw extruder of Werner 30 type (see above) are introduced: an ethylene-methyl acrylate copolymer (methyl acrylate content: 29%, melt index: 3.1), maleic anhydride in a proportion of 0.5% relative to the EMA copolymer, and Luperox 101 in a proportion of 150 ppm relative to the EMA copolymer. The stock temperature is approximately 200xc2x0 C. The residues of free maleic anhydride are removed by degassing in line. The expected polymer is obtained, that is an EMA grafted with 2300 ppm of maleic anhydride and exhibiting a melt index of 2.5, a melting point of 66xc2x0 C. and a Vicat softening point lower than 40xc2x0 C.
The EVA copolymer employed in the examples, marketed by the company Elf Atochem S.A. under the name xe2x80x9cEvatane 2805xe2x80x9d, has a vinyl acetate content of 27-29% by weight, a melt index of 5-8 g/10 min, a melting point of 73xc2x0 C. and a Vicat softening point of 43xc2x0 C.
The SBS block copolymer employed in the examples is the styrene-butadiene-styrene triblock linear copolymer marketed under the name xe2x80x9cCariflex KX 139xe2x80x9d.
The poly-xcex1-methylstyrene resin employed in the examples is the resin marketed by the company DSM under the name xe2x80x9cKristallex F 120xe2x80x9d.
The EVA/MA terpolymer employed in the examples is an ethylene-vinyl acetate-maleic anhydride terpolymer which has a melt index of 7 g/10 min and a vinyl acetate weight content of 26%.
The EBA/MA terpolymer employed in the examples, marketed by Elf Atochem S.A. under the name xe2x80x9cLotader 3700xe2x80x9d is an ethylene-butyl acrylate-maleic anhydride terpolymer which has a melt index of 6 g/10 min and a butyl acrylate weight content of 30%.
Examples 1 to 25 which follow illustrate the preparation of binder compositions for reference and according to the invention (Examples 2 to 25). These compositions have been evaluated for adhesiveness after coextrusion at 220xc2x0 C. of a five-layered sheet with a total thickness of 790 microns and which has the characteristics reported in Table 3, which follows:
For the evaluation of adhesiveness, the peeling is conducted in a laboratory conforming to the standards of a materials evaluation laboratory, according to NFT standard 76-112 part 2, but with the following modifications: a peeling angle of 90 instead of 180 and test piece width of 15 mm instead of 25 mm.
The test pieces are taken from the middle of the coextruded sheets, in the direction of extrusion. The peeling speed is 200 mm/minute. The determination of the peel strengths, on the recordings obtained, is performed according to ISO standard 6133. The reproducibility of the complete sequence (coextrusion/peeling) was verified by employing the same binder a number of times on different days and at different times.